The present invention is directed to an improved method of preparing esters of aryloxyphenoxy propanoic acids. In particular, the present invention is directed to an improved method of preparing esters of pyridyloxyphenoxy propanoic acids. The aryloxyphenoxy propanoic acid esters produced by the present invention are useful are herbicides.
Aryloxyphenoxy propanoic acid esters are generally prepared by a three step process involving the neutralization of hydroquinone followed by coupling with an aryl halide and then reacting the resulting aryloxyphenate with an appropriate halopropionate in an inert solvent at an elevated temperature in the presence of an alkaline material. An excess molar quantity of the halopropionate is usually employed because of competing side reactions. Because an excess of halopropionate is desired, an almost instantaneous addition of halopropionate to the aryloxyphenate is required. This is impractical upon scale-up to commercial production so the aryloxyphenate must be added to the halopropionate. Since the aryloxyphenate must invariably be prepared from the reaction of an aryl halide with hydroquinone due to the lack of commercial availability of aryloxyphenates in large quantities, a second reaction vessel is required so that the aryloxyphenate may be added to the halopropionate.
U.S. Pat. No. 4,046,553 teaches a method of preparing .alpha.-[4-(5-mono-substituted or 3,5-di-substituted-pyridyl-2-oxy)phenoxy]alkanecarboxylic acid esters by reacting a pyridyloxyphenol with a haloalkanecarboxylate in the presence of an alkaline material at a temperature of about 40.degree. C.-120.degree. C. See Column 11, lines 24-47; Column 12, lines 35-57; and Preparation Examples 1 and 4.
British Patent Specification 1,599,121 teaches a method of preparing .alpha.-[4-(5-trifluoromethyl-2-pyridyloxy)phenoxy]alkane carboxylic acid esters by reacting a substituted pyridyloxyphenol with a haloalkanecarboxylate in the presence of an alkaline material at a temperature of 40.degree. C.-200.degree. C. See Page 7, line 30 to Page 8, line 6; Page 8, lines 25 to 36; and Preparation Examples 1 and 3.
U.S. Pat. Nos. 4,214,086 and 4,275,212 teach methods of preparing aryloxyphenols by reacting aryl halides with hydroquinone in the presence of a base such as NaOH or KOH. These reactions result in the formation of water.
U.S. Pat. No. 4,325,729 teaches a method of preparing pyridyloxyphenoxy propionates by reacting a pyridyloxyphenol with an .alpha.-halogencarboxylic acid derivative in the presence of a base. Reaction temperatures are indicated between 0.degree. C.-200.degree. C.
The present methods known to prepare aryloxyphenoxy propanoic acid esters by reacting an aryloxyphenate with a halopropionate, including those described above, suffer from disadvantages, such as, side reactions resulting in the formation of undesirable by-products and a rather low conversion (75-80%) of the starting materials to the desired products. A base, such as, sodium or potassium carbonate, is usually added to the reactants to increase the conversion to 99+% but introduces a solid waste problem. Because of the occurrence of side reactions, an excess of halopropionate is normally employed which introduces purification steps and additional process steps for the recovery of the excess halopropionate.
The present invention remedies the above problems encountered in the preparation of esters of aryloxyphenoxy propionic acid. It has been discovered that water is a major cause of side reactions and is responsible for the low conversion of the starting materials. It has also been discovered that elevated temperatures enhance the formation of a bis(aryloxy)benzene by-product. The formation of this bis by-product is reduced when the third step of the reaction sequence is carried out at a temperature below about 35.degree. C. The combination of a low water level reaction and a low temperature reaction results in advantages, such as: the elimination of the need for additional base; a higher yield based on the propionate starting material; a one vessel reaction; the elimination of the step to recover excess propionate since near stoichiometric amounts of the propionate are employed; and a reduction in the side reactions (by-products) caused by high temperature and high water levels.